flow thru / seniority
The only exception to this rule are the airlines that have "flow through" agreements. IE Continental and Continental Express.
The flow through does not exist any longer between CO and COEX. I believe most majors and their regional partners have either terminated their flow throughs or are heading in that direction. So the answer to the question with respect to seniority is: If you leave company A and start at company B, you will start at company B at the bottom of their list. There may be some exceptions but I am not aware of any. That is the reason that pilots who have time and senioity at their present airline are so reluctant to leave because they know they will be starting at the bottom of the seniority list at their new airline. You can imagine how many pilots at Delta, NW, USAir, United and others are struggling with this very question. It's alot like gambling in Vegas!